Presented by: Hal Groeneboer, Barbara Hawes, Mary Moreira, and Carol Paine.

Introduction
What are the significant differences of the abiotic factors in a old growth forest, secondary forest, and pasture? Las Cruces Biological Station is part of the La Amistad Biosphere Reserve in the southern Talmanca mountain range of Costa Rica. Its elevation ranges from 1,120 to 1,385 meters. Of the 247 hectares, 235 hectares remain as premontaine rain forest, eight hectares are cultivated and four hectares are fallow and experimental plots (Gomez). In an attempt to determine the adaptive significance of deforestation and reforestation, a measure of abiotic factors in a old growth forest, secondary forest and pasture land were measured. Due to the fact that all three areas are subjected to similar climatic conditions, it is predicted that there will be no significant differences in the abiotic factors.

The soils at Las Cruces are primarily 3 types: typic tropohumult, lythic dystropept, and andic humitropept .The soil in the 3 areas we surveyed consisted primarily of the first 2 types. The name of the first type indicates that this soil, found in both the pasture and the second growth forest is a typical tropical humid soil with a significant amount of humus. The soil found in the old growth forest was lythic dystropept which indicates that it is a stony, young tropical soil in which few basic mineral compounds are found.
 

Methodology
Four random sites were chosen in a old growth forest, a secondary forest, and a pasture. Temperature, humidity, penetration, and light factors were measured at each location. Soil samples of approximately two cubic centimeters were taken at each site. A thermometer was placed into the soil to a depth of two centimeters and was allowed to sit for three minutes. A sling psychrometer was used to determine relative humidity. A sharpened pencil was dropped from 1.5 m, pointed end first, into an area of soil, cleared of its top layer of leaf litter. The penetration was then measured to the nearest millimeter. The light meter on a camera was used to determine the relative amount of available light. A camera was focused at a white index card and the f-stop and shutter speed were recorded and used to estimate comparative light levels. A compass was used to assure directional orientation. Samples were brought back to the lab where their pH was tested. Texture of each sample was examined by crude tactile measurements. A sample of 80ml each was weighed and placed in a drying oven for 20 hours @ 105^oC. Five grams of dry soil were crushed and added to 10 ml distilled water. The solution was shaken and allowed to settle for 10 hours. Samples of each were examined under the dissecting microscope to determine particle size.
 

Data
Moist Texture: Old Growth - very crumbly, high organic matter, finer grit

Secondary - clay body with high degree of organic matter, very little sand

Pasture - high clay body, some organic matter, very cohesive, gritty

Dry soil profile - Particle size was largest in the Old Growth, than in the Secondary growth and

smallest in the pasture, when viewed under a dissecting microscope.

Sedimentation: Old Growth – Showed clear separation in two samples and no separation in the other two. One sample had one centimeter of material floating above the water.

Secondary Growth – All samples showed an average of .75 cm of clean water above the sediment.

Pasture –These specimens showed an average of 1.25 cm of very cloud liquid above the sediment.

WET MASS, DRY MASS, DENSITY & PERCENT MOISTURE

Soil sample g /80ml wet g/80ml dry Density wet Density dry % moisture

OG1 39 17 0.4875 0.2125 56%

OG2 60 28 0.75 0.35 53%

OG3 51 30 0.6375 0.375 41%

OG4 50 26 0.625 0.325 48%

2F1 50 27 0.625 0.3375 46%

2F2 37 17 0.4625 0.2125 54%

2F3 44 20 0.55 0.25 55%

2F4 50 28 0.625 0.35 44%

P1 76 38 0.95 0.475 50%

P2 64 36 0.8 0.45 44%

P3 79 39 0.9875 0.4875 51%

P4 60 31 0.75 0.3875 48%

PENETRATION, PH & LIGHT

Soil sample Penetration in cm pH Light

OG1 2 4 2

OG2 2.5 4 2

OG3 2.6 4 4

OG4 3 4 4

2F1 2.5 5 5

2F2 2 5 5

2F3 2.5 5 4

2F4 1.8 4.5 4

P1 1.5 4.5 250

P2 2 4 125

P3 2 4 125

P4 1.5 4.5 125
 
 

SITE AVERAGE COMPARISONS

Average g/80mL wet g/80mL dry Density wet Density dry % moisture

Old Growth 50.0 25.3 0.63 0.32 50

Secondary 45.3 23.0 0.57 0.29 50

Pasture 69.8 36.0 0.87 0.45 48

Average Ground temp. Humidity Light pH Penetration cm

Old Growth 65 78 3.00 4 2.53

Secondary 65 95 4.50 5 2.20

Pasture 75 75 156 4.3 1.75
 
 
 

Discussion
The most noticeable differences include soil density, penetration, light, temperature, and humidity. The pasture had total exposure to sun which caused the higher temperature and lower humidity. The high humidity level in the secondary forest is possibly the result of measurement error. The higher density and compaction in the pasture is probably due to a lower amount of humus in the soil as shown in the sedimentation samples. The pasture sedimentation had minute suspended particles clouding the water. The secondary and old growth forests contained clear water with little or nocclay particles in suspension.

Soil records indicate that the pasture and secondary growth forest are located on similar soil types and therefore should have similar characteristics. However, these same records show the pasture land as forest, so the humus layer may have disappeared after the forest was changed to pasture land. The records used had no dates so it is not known when the pasture land we sampled was converted from forest.

Although there was some variation in pH, these may have been due to level of accuracy in instrumentation.

The significance of these findings have shown our problem statement/hypothesis to be invalid. There are noticeable abiotic differences resulting from several variables at each site. The implication of these findings indicate that deforestation could result in soil that is more compact and dense with less humus, possibly making it less supportive of a diverse ecosystem.
 

Classroom Application
This project could be easily replicated in the classroom using different sites in the school environment such as disturbed vs. undisturbed locations, or cultivated vs. uncultivated, or near industrial sites. This could also be used to indicate flora health and soil type. This also could lead to a study of the importance of soil characteristics in affecting biodiversity in the ecosystem. The teacher could encourage students to question other variables that might exist in different environments. Links could be made to such historical events as the Dust Bowl, or more immediate local environmental concerns such as erosion.
 

References
Donahue, Roy L., John C. Shickluna and Lynn S. Robertson. Soils, An Introduction to Soils and Plant Growth. Englewood Cliffs, N.J.: Prentice-Hall, 1971.

Forsythe, Warren M. "Soil-water Relations in Soils Derived from Volcanic Ash of Central America," Soil Management in Tropical America. Raleigh, N.C.: North Carolina State University, 1975.

Gomez, Luis Diego on the Internet at: http://ots.ac.cr/en.lascruces/

Jones, Jacquelyn c. and John D. Reynolds. "Environmental variables, " Ecological Census Techniques. Cambridge, UK: Cambridge University Press, 1996.